When semiconductor wafers, optical components, etc. are to be made thin, it is common to process them by a so-called rear surface grinding method wherein a circuit surface of a wafer or a surface to be processed of an optical component is protected by a surface protecting sheet, and the protected wafer or component is temporarily fixed to a substrate via the surface protecting sheet, and the rear surface opposite to the circuit surface is ground.
At present, the wall thickness of a wafer is usually 150 μm at an industrial level, however, a thinner wafer is desired. When a wafer is to be made thinner, a phenomenon wherein the ground surface (the rear surface) tends to be non-uniform by an influence of irregularities of the circuit surface, i.e. a rear surface transfer phenomenon of the circuit pattern, tends to be distinct.
The cause for such a rear surface transfer phenomenon may be explained as follows. A pressure sensitive adhesive surface-protecting sheet presently available is limited in its conformability to circuit irregularities of a semiconductor wafer. Therefore, there will be spaces (air pockets) between the adhesive layer and the circuit surface, and the wafer at such regions are not directly supported by the adhesive layer (protective layer). When the wafer is to be made very thin by grinding, at the non-supported scribe line (street), the wafer tends to move in a vertical direction between the circuit and the die while compressing the air pocket, and consequently, such a region will not be ground and will be thicker than other portions. On the other hand, in a case where a hard projection such as a bump is present, the wafer will be ground more, and consequently, such a portion will be thinner than other portions.
Such a phenomenon will not be problematic when the finishing thickness of the wafer is at least 150 μm. However, in a case where the wafer is to be made thinner than 100 μm (particularly when it is to be finished to have a thickness of at most 50 μm) or in a case where a projection such as a bump on the wafer circuit surface is very large (e.g. when it is at least 100 μm), not only the deflecting strength of the wafer is thereby substantially decreased, but also the wafer may be broken during the grinding in an extreme case.
In a case where a wafer is to be ground to be thin at a level of about 50 μm, chipping of the edge of the wafer or penetration of the grinding water between the wafer and the surface protecting layer will, for example, be problematic, and the cause is again due to a defect in adhesion of the surface protecting sheet to the edge of the wafer. Further, with a semiconductor wafer having a projection of at least 100 μm represented by a bump on the circuit surface, not only the thinning of the wafer, but also grinding itself tends to be difficult which is carried out by bonding a typical semiconductor surface protecting sheet.
A conventional surface protecting sheet is usually a sheet having an adhesive layer as a surface protecting layer on a polymer film material. The adhesive is designed so that it has a low elastic modulus so that it will conform to irregularities of the circuit surface. However, if such a tendency is too much, when the sheet is to be peeled off from the wafer, a large stress will be exerted to the wafer, thus leading to breakage.
Accordingly, an energy ray-facilitated release type protecting sheet has been developed whereby before peeling the sheet, the adhesive is cured by irradiation with energy rays such as ultraviolet rays to decrease the bond strength between the wafer and the protecting sheet. However, if the adhesive layer is in a non-cured state during the grinding, there is a problem that it is so flexible that the wafer will be broken during the grinding.
The Patent Document 1 discloses a method for grinding a wafer wherein an energy ray-facilitated release type protecting sheet as described above, is bonded on a wafer having a circuit formed, and the adhesive layer is cured by energy rays, whereupon grinding of the rear surface of the wafer is carried out. However, the adhesive is not a fluid, whereby the conformability to irregularities on the wafer circuit is surface is not sufficient.
Patent Document 2 discloses a hot melt type semiconductor surface-protecting sheet. The hot melt type sheet to be melted to show fluidity when heated to a level of from 60 to 100° C., conforms to irregularities on the circuit surface to present excellent grinding properties. However, this sheet has such a nature that it melts every time when the temperature becomes higher than the melting point.
On the other hand, a semiconductor wafer may usually be subjected to e.g. a heating step between a step of bonding a die attachment film (hereinafter referred to also as “DAF”) i.e. a film to be used for fixing a chip after the wafer is bonded to the protecting sheet and a step of forming a metal film by sputtering. Therefore, by the temperature rise in such a step, there has been a trouble such that the protecting sheet will be melted again.
Patent Document 1: JP-A-11-026406
Patent Document 2: JP-A-2000-38556